1. Field of the Invention
This invention generally relates to a hydrostatic pump with a variable displacement volume which can be operated in a closed circuit. The pump has a mechanical displacement volume control with a mechanical control element. The mechanical control element comprises a piston-controlled control valve for the generation of a control pressure that acts on a positioning piston, which is functionally connected with a displacement volume setting device, in particular a swashplate. The position-controlled control valve is functionally connected with a control lever and with the displacement setting device of the pump.
2. Technical Considerations
Hydrostatic pumps of this general type with variable displacement volumes which are operated in a closed circuit are used, for example, as pumps in traction drives of mobile work machines.
On mechanical displacement volume control systems of pumps of the known art, it has not been possible to provide a pressure cutoff function because there is a positive mechanical coupling between the control lever and the displacement setting device formed by the swashplate.
On a known pump illustrated in FIGS. 1a to 1c having a mechanical displacement volume control, the control lever 30 is coupled with a cam plate 31 which is provided with an arc-shaped groove 41 in which one end of an actuator lever 42 is engaged by means of a pin 43. The actuator lever 42 is also in functional communication with a spool element of a position-controlled control valve 6 for its actuation. The displacement setting device, which is in the form of a swashplate, is also in functional communication by means of an assembly 44 with the actuator lever 42, for example, by means of a pin 45. The actuator lever 42 also affects a positive mechanical coupling between the control lever 30 and the displacement setting device to achieve a mechanical emergency actuation of the displacement setting device by means of the control lever 30 in the event that the supply pressure of the control valve 6 fails.
For this purpose, on the second end of the actuator lever 42 there is a notch 46 in which the assembly 44 connected with the displacement setting device is engaged with a defined amount of clearance by means of a pin 47. If the control pressure feed to the control valve 6 fails, when the control lever 30 is actuated by means of the cam plate 31, the actuator lever 42 is pivoted around the pin 45 of the assembly 44 so that a defined clearance between the notch 46 and the pin 47 of the assembly 44 is closed. The notch 46 of the actuator lever 42 therefore comes into contact with the pin 47, as a result of which the actuator lever 42 is connected via the assembly 44 directly with the displacement setting device so that there is a positive mechanical coupling between the actuator lever 42 and the displacement setting device. When the control lever 30 is actuated and thus the cam plate 31 is actuated via the actuator lever 42, the displacement setting device (such as a swashplate) can be pivoted directly mechanically.
With a pressure cutoff function, during the acceleration phase, when a maximum pressure is reached in a delivery line of the pump, the pump is controlled to reduce the displacement volume. As a result, a response by the overpressure protection device of the closed circuit, such as a pressure relief valve, is prevented. This measure prevents hydraulic fluid from flowing out of the circuit via the overpressure protection device, which would result in power losses as well as an unnecessary heating of the hydraulic fluid. As a result of the mechanical positive coupling between the control lever and the displacement setting device in the pump with a mechanical displacement volume control of the known art illustrated in FIGS. 1a to 1c, if the displacement setting device in the form of a swashplate were to pivot back, the control lever would also be actuated. This means that a pressure cutoff function cannot be provided with the mechanical displacement volume control of the known art.
Therefore, it is an object of this invention to provide a pump with a mechanical displacement volume control which is provided with a pressure cutoff function with little construction effort.